How do vaccines work? Vaccines contain weakened or inactive parts of a particular organism (antigen) that trigger an immune response within the body. If the body encounters the real antigen later, it already knows how to defeat it. Frank Augstein/AP How are mRNA vaccines different? Rather than having the viral protein inside the vial, an mRNA vaccine contains the “instructions” for our own bodies to make the vaccine. The body translates these instructions and teaches our cells how to make a harmless piece of what is called the “spike protein”, found on the surface of the virus that causes Covid-19.